Japanese researchers found eating the yoghurt reduced levels of hydrogen sulphide - a major cause of bad breath - in 80% of volunteers.

The key are active bacteria in yogurt, specifically Lactobacillus bulgaricus and Streptococcus thermophilus.

Details were presented at a meeting of the International Association for Dental Research.

A total of 24 volunteers who took part in the study were given strict instructions on oral hygiene, diet and medication intake.

They spent two weeks avoiding yoghurts and similar foods, like cheese.

Researchers then took saliva and tongue coating samples to measure bacteria levels and odour-causing compounds, including hydrogen sulphide.

The volunteers then ate 90 grams of yoghurt a day for six weeks.

At the end of the study, researchers took samples again. They found hydrogen sulphide levels decreased in 80% of participants.

Levels of plaque and the gum disease gingivitis were also significantly lower among yoghurt eaters.

Added bonus

Dr Nigel Carter, chief executive of the British Dental Health Foundation, said: "The foundation has long been drawing people's attention to sugar-free yoghurts as a healthy snack, so it is pleasing to hear that it may have oral health benefits we were previously unaware of.

"Frequent consumption of sugary snacks is the principal cause of tooth decay, which can cause a great deal of pain and discomfort.

"Although this research is still in the early stages there is no doubt that sugar-free yoghurts provide a much healthier alternative to sweets and chocolate, and we would encourage snackers to incorporate them into their diet."

One in four people suffer from bad breath regularly, while 19 in 20 are affected by gum disease at some point in their lives.

However, Dr Carter stressed that the best way to beat bad breath was by adopting a good oral health routine.

This involves brushing twice-a-day with fluoride toothpaste, cutting down on the frequency of sugary snacks and drinks, flossing daily and visiting a dentist regularly.